The documentation on Create a link to start a new topic with pre-filled information says:
However, even when directly copy-pasting the listed example, a redirect appears to strip one of the slashes from the URL specified after
title= , resulting in the browser instead navigating to
https://meta.discourse.org/new-topic?title=https:/www.xkcd.com/556 which opens a composer window for a new topic with the pre-filled title
https:/www.xkcd.com/556. The behavior and oneboxing described in Start a topic by pasting a link (like Reddit) then obviously doesn’t occur, unless you manually add another
/ back in to the title URL.
I first saw this behavior trying things out on my hosted Discourse and figured I might have something mis-configured, but then seems like a bug since it also happens with the provided example?